This invention relates to radiolocation, as of mobile vehicles such as aircraft equipped with transponders, and more particularly to improvements on the invention described in U.S. Pat. No. 4,115,771. The system of the '771 patent provides essentially all the information that would be available with an actual secondary surveillance radar (SSR) located at the site of the virtual secondary surveillance radar (VSSR), without transmitting any additional signals, and at a fraction of the cost of an actual SSR. Under certain geometrical conditions, the information regarding range of a target craft from the VSSR can become inaccurate or indeterminate. One solution to this problem is described in U.S. Pat. No. 4,899,156. However, the system of that patent requires occasional radar transmissions in addition to those of the standard air traffic control radar beacon system (ATCRBS).